In modern integrated circuits, there is a need for more voltage levels than are supplied on the printed circuit board. For example, a DRAM with a negative wordline scheme will require the wordline voltage (e.g. -0.5V) and also a negative substrate bias (e.g. -0.75V). Positive voltages will include the boosted wordline voltage (e.g. 3.3V), the internal voltage (e.g. 2.5V), the bit-line high voltage (e.g. 2.1V), etc.
Some of the power supply levels will need to handle much more current than others. The high-current levels will ramp up quickly during a power-on period, while the levels that need only low current will have charge pumps with a smaller capacity in order to save power and will take longer to reach their level, even if that level has a smaller absolute value than the high-capacity level.
The lower capacity pumps could be redesigned with increased capacity so that it will improve the ramp-up speed, but that would waste silicon area, and consume more power.